1. Field of the Invention
The present invention generally relates to radio frequency plasma processing reactors, especially as used in semiconductor processing and, more particularly, to high throughput reactors capable of achieving high etch rates with high material selectivity with reduced damage to extremely thin layers of material.
2. Description of the Prior Art
Processing of semiconductor materials to form integrated circuits has become highly sophisticated and is presently capable of producing very high performance circuit elements at very small feature size regimes and extremely high integration density. As devices are scaled to small sizes manufacturing and process tolerances are also reduced and structures formed must be of increasingly exact dimensions in order to provide desired electrical characteristics. Further, many types of structures become much more subject to damage during manufacturing processes. Moreover, manufacturing processes increasingly rely upon selectivity between materials to form structures of sub-lithographic dimensions and to maintain independence between processes so that results of particular processes are confined to the intended structures.
In particular, as metal-oxide-semiconductor (MOS) field effect transistors and capacitors are scaled to smaller sizes, the thickness of oxide insulators must be reduced to a thickness of often much less than 80 Angstroms. Such a thin structure is particularly subject to damage from receiving charge build-up thereon which can cause breakdown and damage to the thin oxide film, particularly from charged particles produced in plasma processes such as reactive ion etching (RIE) of other structures. Such charge build-up can occur through several mechanisms such as insufficiently complete neutralization of surface charge, non-uniform plasma density and potential, electrons diffusing out of the plasma when the electron temperature is too high and collecting on the top surface of high aspect ratio structures and/or excessive RF bias on the workpiece.
Unfortunately, plasma processes have such advantages in terms of predictability, repeatability and throughput that they remain the process of choice notwithstanding the increased likelihood of oxide damage. Accordingly, the plasma conditions must be closely chosen and regulated to reliably form structures without loss of significant manufacturing yield to oxide damage. Such regulation of plasma conditions is often inconsistent with economically acceptable levels of plasma reactor throughput.
Numerous ways are known for producing a plasma in a plasma reactor vessel or tool. However, particle interactions within a plasma are very complex and the production of some desirable plasma conditions are often linked with other undesirable conditions in various known plasma processes such that the desirable condition cannot be independently achieved.
For example, known RF plasma systems do not produce sufficient plasma density to support production throughput levels without becoming a direct source of oxide and device damage. Further, these systems produce an excessive electron temperature near the wafer which can also cause damage. Modification of these systems by the addition of magnetic fields to concentrate the plasma have resulted in damaging non-uniform plasmas.
Electron cyclotron resonance (ECR) plasma systems have been used and provide an increase in plasma density but also produce non-uniform plasmas and excessive electron temperatures near the wafer. More recently, inductively coupled plasma (ICP) systems, as disclosed in U.S. Pat. Nos. 4,948,458 and 5,304,279, have been used to produce plasmas of sufficient uniformity and density but with insufficient selectivity, particularly for oxides and low dielectric constant (xe2x80x9clow Kxe2x80x9d) materials, due to the production of additional undesired radicals from the etchants required for oxides, referred to as gas cracking. ECR plasma systems also produce significant levels of oxide and device damage.
U.S. Pat. Nos. 5,565,738 and 5,707,486 disclose VHF/UHF plasmas using power at frequencies in excess of about 40 MHz. These plasmas have the advantage of producing a lower electron temperature plasma with an enhanced high energy xe2x80x9ctailxe2x80x9d in the energy distribution of the electrons produced which leads to increased ionization for the same or a reduced level of gas cracking. However, these plasma sources produce VHF/UHF voltages in the plasmas thus producing plasma density variation due to ionization in undesired locations within the reactor vessel; both of which are sources of damage to oxides and devices.
U.S. Pat. No. 5,783,102 describes production of a cooler electron reactive plasma and negative ion plasmas by using an internal magnetic filter to separate the hot electron plasma near the plasma generation location in the plasma reactor from cooler electron plasma more proximate to the workpiece or wafer. However, this technique results is an unacceptable reduction of ion current to the wafer and the internal magnets within the reaction vessel can be a source of wafer contamination.
It has been shown in the field of neutral beam injection using negative hydrogen sources that a large magnetic field applied externally to the extractor as a filter can reduce electron density and temperature. However, these systems have a rectangular geometry which is undesirable for a plasma processing reactor. Using diode ring magnets to obtain a desired circular geometry produces non-uniform plasma density, and potential and non-uniform electron temperature as well, and thus produce both damage and non-uniform etching.
In summary, known plasma processing reactors and plasma production and confinement techniques have not heretofore been capable of producing a substantially uniform plasma at the workpiece or wafer with sufficiently high plasma density to support production throughput levels while being highly selective and avoiding damage, particularly of oxides and low K materials. Therefore, uniform etching of oxides and insulators selectively to underlying layers cannot be reliably achieved at production throughput levels without significant reduction of manufacturing yield due to damage from charging effects of the plasma.
It is therefore an object of the present invention to provide a plasma processing reactor capable of producing a highly uniform plasma of increased plasma density and reduced electron temperature.
It is another object of the invention to provide a plasma etching process capable of reliably etching insulator films with high selectivity and limited, if any, damage.
It is another object of the invention to provide an antenna and/or a magnetic filter suitable for retrofitting existing reactor vessels to improve uniformity of plasma processes while increasing throughput and manufacturing yield.
In order to accomplish these and other objects of the invention, an antenna and a plasma tool including the antenna are provided, the antenna including a plurality of elements, and an arrangement including a delay line for applying VHF/UHF power to respective antenna elements with respective phase differences such that a majority of RF current to the plasma is returned to other respective antenna elements and voltages in said plasma are substantially canceled.
In accordance with another aspect of the invention, a magnetic filter and plasma tool including the magnetic filter are provided for separating a hot plasma region from a cold plasma region, wherein the magnetic filter is located outside a cavity of the plasma tool and further includes an arrangement for providing a preferential drift path for charged particles connecting the hot plasma region and the cold plasma region.
Either or both of the antenna and the magnetic filter may be retrofitted to existing plasma reactor vessels and provide increased plasma density to improve process throughput and high plasma uniformity to improve uniformity of etching or other processing by returning VHF/UHF RF currents to the antenna and cancellation of voltages in the plasma within the plasma while maintaining material selectivity of the plasma process by avoidance of gas cracking and avoiding oxide and device damage by improved confinement and segregation of hot and cold electrons. Thus, use of the antenna or the magnetic filter or both in accordance with the invention, even in existing plasma tools, provides a combination of meritorious effects not possible prior to the invention.